Method and installation for laser welding with a N2/He gas mixture, the N2/He content being controlled according to the laser power

ABSTRACT

A method and an apparatus for welding with a laser beam. A shielding gas mixture of nitrogen and helium is used and the proportions of the component gases are modified depending on the laser beam&#39;s power or power density. Plasma formation in the shield gas is minimized by increasing the proportion of helium as the laser beam&#39;s power or power density increases.

BACKGROUND

The present invention relates to a laser beam welding process using agas mixture consisting of nitrogen and helium in proportions that areadjusted or adapted according to the power or power density of the laserdevice used.

In industry, it is known to use a laser beam to cut or weld one or moremetal workpieces. In this regard, the following documents may be cited:DE-A-2 713 904, DE-A-4 034 745, JP-A-01048692, JP-A-56122690, WO97/34730, JP-A-01005692, DE-A-4 123 716, JP-A-02030389, U.S. Pat. No.4,871,897, JP-A-230389, JP-A-62104693, JP-A-15692, JP-A-15693,JP-A-15694, JP-A-220681, JP-A-220682, JP-A-220683, WO-A-88/01553,WO-A-98/14302, DE-A-3 619 513 and DE-A-3 934 920.

Laser welding is a very high-performance welding process as it makes itpossible to obtain, at high speeds, very great penetration depthscompared with other more conventional processes, such as plasma welding,MIG (Metal Inert Gas) welding or TIG (Tungsten Inert Gas) welding.

This is explained by the high power densities involved when focusing thelaser beam by one or more mirrors or lenses in the joint plane of theworkpieces to be welded, for example power densities that may exceed 10⁶W/cm².

These high power densities cause considerable vaporization at thesurface of the workpieces which, expanding to the outside, inducesprogressive cratering of the weld pool and results in the formation of anarrow and deep vapor capillary called a “keyhole” in the thickness ofthe plates, that is to say in the joint plane.

This capillary allows the energy of the laser beam to be directlydeposited depthwise in the plate, as opposed to the more conventionalwelding processes in which the energy deposition is localized on thesurface.

This capillary is formed from a metal vapor/metal vapor plasma mixture,the particular feature of which is that it absorbs the laser beam andtherefore traps the energy within the actual capillary.

One of the problems with laser welding is the formation of a shieldinggas plasma.

This is because this metal vapor plasma, by seeding the shielding gaswith free electrons, may induce the appearance of a shielding gasplasma, which is prejudicial to the welding operation.

The incident laser beam may therefore be greatly, or even totally,absorbed and therefore may lead to a substantial reduction in thepenetration depth, or even in a loss of coupling between the beam andthe material and therefore a momentary interruption in the weldingprocess.

The power density threshold at which the plasma appears depends on theionization potential of the shielding gas used and is inverselyproportional to the square of the wavelength of the laser beam.

Thus, it is very difficult to weld under pure argon with a CO₂-typelaser, whereas this operation may be carried out with very much less ofa problem with a YAG-type laser.

In general, in CO₂ laser welding, helium is used as shielding gas, thisbeing a gas with a high ionization potential and making it possible toprevent the appearance of the shielding gas plasma, and to do soirrespective of the laser beam power employed.

However, helium has the drawback of being an expensive gas and manylaser users prefer to use other gases or gas mixtures that are lessexpensive than helium but which would nevertheless limit the appearanceof the shielding gas plasma and therefore obtain welding results similarto those obtained with helium, but at a lower cost.

Thus, gas mixtures are commercially available that contain argon andhelium, for example the gas mixture containing 30% helium by volume andthe rest being argon, sold under the name LASAL™ 2045 by L'Air Liquide™,which make it possible to achieve substantially the same results ashelium, for CO₂ laser power levels below 5 kW and provided that thepower densities generated are not too high, that is to say above about2000 kW/cm².

However, the problem that arises with this type of Ar/He mixture is thatit is no longer suitable for higher laser power densities, since thethreshold at which the shielding gas plasma is created is then exceeded.

It is an object of the present invention therefore to solve this problemby proposing an improved laser welding process that can employ laserswith a power exceeding 15 to 20 kW and to do so, with no or minimalshielding gas plasma formation, irrespective of the power or powerdensity chosen.

SUMMARY

The solution of the invention is therefore a laser beam welding processemploying a shielding gas mixture containing nitrogen and helium, inwhich the proportion of nitrogen and/or helium in said gas mixture ischosen or adjusted according to the power or power density of said laserbeam.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 illustrates threshold levels for the appearance of plasma in theshielding gas, as a function of the helium content in nitrogen and ofthe laser's power density; and

FIG. 2 illustrates threshold levels for the appearance of plasma in theshielding gas, as a function of the helium content in nitrogen and ofthe laser's power.

FIG. 3 illustrates the workpiece being welded according to an aspect ofthe disclosed method.

DESCRIPTION OF PREFERRED EMBODIMENTS

The solution of the invention is therefore a laser beam welding processemploying a shielding gas mixture containing nitrogen and helium, inwhich the proportion of nitrogen and/or helium in said gas mixture ischosen or adjusted according to the power or power density of said laserbeam.

Depending on the case, the process of the invention may include one ormore of the following technical features:

-   -   the laser power is between 0.5 kW and 30 kW, preferably between        5 kW and 20 kW;    -   the shielding gas mixture consists of nitrogen and/or helium;        preferably, the gas mixture contains a helium volume proportion        of 30% to 80%, the remainder being nitrogen and possibly        inevitable impurities;    -   the gas mixture is produced on site, by mixing defined amounts        of nitrogen and helium;    -   the gas mixture is produced by means of a gas mixer system        slaved to the laser power or power density employed so as to mix        controlled amounts of nitrogen and helium;    -   the proportion of helium in the gas mixture is increased when        the laser power or power density is increased.

According to another aspect, the invention also relates to a laser beamwelding process employing a shielding gas mixture containing helium andnitrogen, in which the proportion of helium relative to the proportionof nitrogen in said gas mixture is chosen or adjusted according to thepower or power density of said laser beam so as to avoid or minimizeplasma formation in the shielding gas during welding.

According to another aspect, the invention also relates to a laser beamwelding process employing a shielding gas mixture containing helium andnitrogen, in which the volume proportion of helium in said gas mixtureis:

-   -   between 1 and 30% for a laser beam power of between 0.5 kW and 4        kW;    -   between 30 and 50% for a laser beam power of between 4 kW and 8        kW; and/or    -   between 50 and 70% for a laser beam power of between 8 kW and 12        kW.

Moreover, the invention also relates to a laser beam welding processemploying a shielding gas mixture containing helium and nitrogen, inwhich the volume proportion of helium in said gas mixture is:

-   -   between 1 and 30% for a laser beam power density of between 500        kW/cm² and 2000 kW/cm²;    -   between 30 and 50% for a laser beam power density of between        2000 kW/cm² and 4000 kW/cm²; and/or    -   between 50 and 70% for a laser beam power density of between        4000 kW/cm² and 10000 kW/cm².

The helium and nitrogen preferably come from a single gas source inwhich the helium and nitrogen are premixed in the desired proportions,for example, by means of a gas mixer.

The invention also relates to a laser beam welding installationemploying a shielding gas mixture containing nitrogen and helium,comprising:

-   -   at least one nitrogen source;    -   at least one helium source;    -   gas mixing means allowing the nitrogen coming from the nitrogen        source to be mixed with the helium coming from the helium        source;    -   a laser generator device delivering a laser beam having a laser        power of at least 0.5 kW; and    -   regulating means that cooperate with said gas mixing means so as        to adjust the proportions of nitrogen and/or helium according to        the laser power delivered by the laser device.

Furthermore, the invention also relates to a laser beam welding processemploying a shielding gas mixture containing helium and nitrogen, inwhich the volume proportion of helium (% He) in said gas mixture as afunction of the power density is such that:28×In(Φ_(P))−207≦% He≦32.3×In(Φ_(P))−207in which:

-   -   In(Φ_(P)) represents the natural logarithm of the power density        expressed in kW/cm²; and    -   % He represents the volume percentage of helium in nitrogen of        the gas mixture.

Preferably, the volume proportion of helium (% He) in said gas mixtureas a function of the power density is such that:28.5×In(Φ_(P))−207≦% He≦31.5×In(Φ_(P))−207.

Also preferably, the volume proportion of helium (% He) in said gasmixture as a function of the power density is such that:29×In(Φ_(P))−207≦% He≦31×In(Φ_(P))−207.A greater understanding of the invention will now be gained from theexplanations given below with reference to the appended figure.

As explained above, in laser beam welding, a major problem that arisesis associated with the creation of a shielding gas plasma harmful to thewelding operation by the strong, or even total, absorption of the laserbeam that it generates, and thus results in an appreciable reduction inthe penetration depth, or even in a loss of coupling between the laserbeam and the material to be welded, and therefore in an interruption inthe welding process.

Now, the inventors of the present invention have demonstrated that thethreshold for the appearance of the shielding gas plasma is determined,for a given CO₂-type laser power density, by the volume proportion ofhelium (relative to that of nitrogen) in the helium/nitrogen gas mixtureused as shielding gas during the welding operation and that thisproportion of helium has to be varied according to the power density ofthe laser.

Thus, FIG. 1 shows (curve A) the change in the threshold appearance ofplasma as a function of the power density (plotted on the x-axis) and ofthe volume proportion of helium (plotted on the y-axis) in the mixtureformed from nitrogen and helium, the sum of the nitrogen and heliumcontents constituting 100% by volume of the mixture.

Curve A was obtained by analysis of the depth of penetration of the weldbeads produced with various helium contents in the mixture and by visualexamination of the appearance, or nonappearance, of the shielding gasplasma during the welding process.

The power density was obtained by dividing the laser power on theworkpiece by the diameter of the focal spot obtained with the laser inquestion, measured beforehand by means of a laser beam analyzer.

The region lying above curve A represents the region in which, for thepower density in question, the helium content in the nitrogen allows aweld bead to be produced without shielding gas plasma appearing.

In the region lying below curve A, the shielding gas breaks down andtherefore a shielding gas plasma is present.

In order to indicate the uncertainties associated with measuring thediameter (in microns) of the focal spot, with that of the helium contentin the nitrogen/helium mixture and with that regarding the energydistribution within the focal spot, three bundles of curves (B, C), (D,E) and (F, G) have also been shown in FIG. 1.

The equations of these curves are of the type:% He=μ×In(ψ_(P))−207in which:

-   -   In(Φ_(P)) represents the natural logarithm of the power density        expressed in kW/cm²;    -   % He is the helium percentage in the nitrogen; and    -   μ is a value that depends on the curve in question: μ=31 for        curve B; μ=29 for curve C; μ=31.5 for curve D; μ=28.5 for curve        E; μ=32.3 for curve F; and μ=28 for curve G.

Thus, within the area of the drawing lying between curves F and G (oralternatively D and E or B and C), it is possible to choose, for thepower density in question, the N₂/He mixture that makes it possible toobtain the same performance as either pure helium or as a N₂/He mixturelocated above the region lying between curves F and G (or alternativelyD and E or B and C).

Conversely, below this region the shielding gas always breaks down andtherefore a shielding gas plasma appears. The gas mixture determinedfrom these curves is therefore the optimum mixture, that is to say theone which contains the least helium but which gives, however,substantially the same results as pure helium or as a mixture with ahigher proportion of helium.

All these curves were produced at a welding speed of 3 m/min on steeland stainless steel workpieces with a parabolic mirror of 250 mm, 200 mmor 150 mm focal length, and using a CO₂ laser whose Q-factor was 4.

As shown in FIG. 1, a helium/nitrogen mixture containing 50% by volumeof nitrogen gives penetration depths and welding speeds that areapproximately the same as for pure helium for a CO₂ laser power densityof 5.3×10⁶ W/cm².

The invention may also be demonstrated by showing the change in thethreshold for the shielding gas plasma to appear as a function of thehelium content in nitrogen and of the laser power employed, as showndiagrammatically in FIG. 2.

This other representation, less general than the previous one, may beobtained from the curves of FIG. 1 and using the following equations:φ_(P) =P/S  (1)where Φ_(P) is the power density, P is the laser power used and S is thearea of the focal spot;S=πW₀ ²  (2)where W₀ is the radius of the focal spot; andW ₀ W _(F) =M ²(λf/π)  (3)where W_(F) is the radius of the laser beam at the mirror or at thefocusing lens for the power in question, M² is the Q-factor of the laserbeam, which in general is a manufacturer's datum (M²=1 for a Gaussianbeam), λ is the wavelength of the laser beam (10.6 μm in the case of aCO₂ laser) and f is the focal length of the mirror or of the focusinglens.

Thus, it is possible to switch from a power density representation(FIG. 1) to a power representation (FIG. 2), and vice versa, using theabove equations, in order to determine on the basis of the power orpower density used the corresponding nitrogen/helium mixture.

FIG. 2 was obtained in this case from the curves of FIG. 1, and for aQ-factor of 4, a focal length of 200 mm and a beam diameter at thefocusing mirror of 28 mm.

Thus, at 6 kW, with a focal length of 200 mm, for a laser of 4 Q-factorand a beam diameter at the focusing mirror of 200 mm, it is possible touse a nitrogen/helium mixture containing 50% by volume of each of thesecomponents.

The present invention is therefore based on the fact that the N₂/He gasmixture is adapted or adjusted according to the laser power or powerdensity used in order to obtain a high-quality weld and for reducedcost, without shielding gas plasma generation or else with as littleplasma generation as possible.

According to the invention, the proportions of the components in the gasmixture may be adjusted on the basis of the volume, molar or massproportions; however, a volume adjustment is preferred as it is simplerto implement.

Starting from this basis, the invention may be implemented by producinga range of gas mixtures in bottles, that is to say in packaged form,with a variable helium content in the nitrogen adapted according to thelaser power or power density.

For example, the table below gives three different N₂/He mixturesadapted to three respective ranges of laser power density recommendedfor implementing the invention.

Composition of the N₂/He gas mixture (expressed as Recommended laserpower vol % of He) density ranges N₂ + 30% He 500 to 2000 kW/cm² N₂ +50% He 2000 to 4000 kW/cm² N₂ + 70% He 4000 to 10 000 kW/cm²

Depending on the case, the invention may also be used directly on siteby an operator, before the start of welding, for example on the basis ofa source of helium and nitrogen, the N₂/He gas mixture most suited tothe power density or to the power of the laser used, and according tothe specifications of the figure appended hereto.

Alternatively, and as best illustrated by FIG. 3, the desired N₂/Hemixture may also be obtained by automatic slaving of the gas mixer 4according to the power or power density of the laser 2 used and by usingthe curve of the figure appended hereto as calibration curve, whereinlaser 2 directed a laser beam 3 towards a workpiece to be welded 1.

The laser welding process of the invention is particularly suitable forwelding workpieces made of aluminum or aluminum alloys, stainless steelor mild steel.

The laser welding process of the invention may be used for weldingworkpieces of the same or different thickness ranges between 0.1 mm and300 mm.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. A method for laser beam welding with a shielding gas mixture, saidmixture comprising helium and nitrogen, wherein the volume proportion ofsaid helium in said mixture comprises about 1% to about 30% for a laserbeam power density of about 500 kW/cm² to about 2000 kW/cm².
 2. A methodof laser beam welding with a shielding gas mixture comprising helium andnitrogen, wherein the volume proportion of said helium in said mixtureis a function of the power density such that:28×In(Φ_(P))−207≦% He≦32.3×In(Φ_(P))−207 wherein: a) In(Φ_(P))represents the natural logarithm of said power density expressed inkW/cm²; and b) % He represents the volume percentage of helium innitrogen of said gas mixture.
 3. The process of claim 2, wherein saidvolume proportion of said helium in said mixture is a function of saidpower density such that:28.5×In(Φ_(P))−207≦% He≦31.5×In(Φ_(P))−207.
 4. The process of claim 3,wherein said volume proportion of said helium in said mixture is afunction of said power density such that:29×In(Φ_(P))−207≦% He≦31×In(Φ_(P))−207.
 5. The method of claim 2,wherein said mixture consists essentially of: a) a helium volumeproportion of about 30% to about 80%; and b) a nitrogen volumeproportion of about 20% to about 70%.
 6. A method of laser beam weldingwith a shielding gas mixture, said mixture comprising helium andnitrogen, wherein the volume proportion of said helium in said mixturecomprises about 30% to about 50% for a laser beam power density of about2000 kW/cm² to about 4000 kw/cm₂.
 7. A method of laser beam welding witha shielding gas mixture, said mixture comprising helium and nitrogen,wherein the volume proportion of said helium in said mixture comprisesabout 50% to about 70% for a laser beam power density of about 4000kW/cm² to about 10000 kw/cm².